The embodiments described herein relate generally to the fabrication, repair, or both, of metals, alloys and superalloys, such as those used in construction of turbine engine diffusers, blades and vanes, heat exchangers, corrosion-resistant vessels and piping, among other component applications. For economy of expression, we refer to all such component-forming materials as “alloys”, where it is clear from context which material or materials are intended. It is often necessary in the fabrication or repair of such alloys to join two pieces of alloy, to repair a crack, or to fill a void, gap or hollow in the alloy. For economy of language, we refer to all such activities as “repair” or “repairing of the alloy.” Although brazing generally involves the brazing of a single type of alloy as the base material that forms the component's substrate (also referred to as the base metal, base alloy or base superalloy), brazing techniques are not inherently limited to a single composition of base material, nor are the embodiments described herein so limited. As is known in the field, dissimilar alloys are often joined by brazing. The range of compositions of the brazing materials described herein permits routine experimentation to learn whether particular dissimilar alloys under consideration can be joined. For economy of language, we describe herein brazing a single type of alloy which forms the component substrate, understanding thereby that joining of dissimilar alloys is not inherently excluded as could be determined by routine testing.
Common methods for repairing alloys include welding with or without added filler material, brazing and soldering. Welding involves melting a region of the base material whereas brazing and soldering involve melting only the filler material but generally not melting the surrounding alloy. Braze filler material is referred to herein as braze filler, braze filler material, braze material or braze alloy interchangeably and without distinction. It is conventional to identify as “brazing” processes in which the liquidus temperature of the filler material is typically above about 450 degrees Celsius, (842 degrees F.) but below the solidus temperature of the base metal substrate. We follow conventional terminology in which the liquidus temperature denotes the boundary between an all-liquid phase and a phase containing both solid and liquid, and solidus denotes the boundary between an all-solid phase and a phase containing both liquid and solid. Soldering relates to the use of filler materials that melt below about 450 deg. C. The chief focus herein relates to brazing.
Joining by brazing of flat or complimentary surfaces (such as the opposing walls of shaped pieces or the opposing faces of some cracks) can often be accomplished with only a thin layer of brazing material between the surfaces to be joined, typically less than about 1 mm, often about 0.1 mm in thickness. Such a thin braze joint generally takes on properties similar to those of the surrounding base alloy. That is, the mechanical properties, the corrosion resistance, the high temperature performance as well as other properties of such a thin braze joint is typically not markedly different from that of the surrounding base alloy substrate. For example, such nearly matching properties might be obtained by short range diffusion brazing often using braze filler materials with the same composition as the base metal, modified only by the addition of small amounts of a melting point suppressant such as boron. In such cases, diffusion brazing can result in a braze filling that virtually matches the base metal composition and properties.
On the other hand, many brazing processes require braze filler material to span a gap larger (sometimes much larger) than about 1 mm, or to fill a substantial gap, void or hollow in the base alloy, hereinafter a “wide gap.” In such cases, the brazed region generally takes on mechanical properties, corrosion resistance, high temperature properties and other properties similar to those of the braze material rather than the properties of the base alloy (hereafter referred to generally as “substrate properties”). Long range diffusion of melting point suppressant from such a wide gap of braze material to achieve properties substantially matching those of the base metal is not normally achievable. Thus, a significant degradation of the performance of such an alloy substrate at, and in the neighborhood of, the brazed portion is the undesirable result. Under the “weakest link” standard, the overall performance of the component including such brazed region is likely to be significantly degraded. For these reasons, brazing repair (especially for wide gaps) is generally considered a non-structural repair, that is, a repair that does not maintain the desired properties throughout the brazed region. Typical conventional braze materials generally have material structural properties significantly less than the properties of the base metal, sometimes less than about 70% of the base metal properties.
One known braze material formulation approach is to use the base material itself as the starting point for formulating a braze filler with suitable modifications to achieve an adequate brazing material. For example, a brazing process requires that the braze material melt at a lower temperature than the base material, more precisely, that the liquidus of the braze filler be less than the solidus of the base material. Thus, melting point suppressants may be added to a base material to produce a braze filler material with depressed melting point. For example, boron (B) is sometimes used to depress braze material melting points in the diffusion brazing of superalloys in an attempt to achieve near base metal properties. However, as noted by Huang (U.S. Pat. No. 8,197,747), boron tends to form brittle hard phases with other alloying elements within the joint or repaired area, thereby reducing the ductility, fatigue life and the corrosion resistance properties of the base material, within the joined or repaired region.
Nickel (Ni) base braze alloys have been proposed having low boron (B) content in an effort to avoid brittleness but with the addition of hafnium (Hf) in order to achieve desired melting point suppression (for example, see Jiang et al U.S. Pat. No. 7,156,280). Such braze materials may reduce problems of brittleness, but do not eliminate them so that further improvement would be desirable. Furthermore, such alloys often introduce other disadvantages. For example, alloys of nickel-hafnium-chromium (Ni—Hf—Cr), nickel-hafnium-cobalt (Ni—Hf—Co) and nickel-hafnium-molybdenum (Ni—Hf—Mo), all without boron, have been proposed by Buschke et al “New Approaches for Joining High-Temperature Materials,” Proceedings from Materials Conference '98 on Joining Advanced and Specialty Materials, pp. 51-55, 12-15 Oct. 1998, Rosemont, Ill., by M. Singh, J. Indacochea, D. Hauser Eds. (Published by ASM International, Materials Park, Ohio 44073-0002). However, such alloys often have additional disadvantages. For example, such brazing alloys typically require relatively high brazing temperatures of the order of about 1235 deg. C. Furthermore, the absence of B, Si (silicon) or both from the brazing alloy can cause the alloy to exhibit poor wettability. That is, when the braze alloy is heated to or above its melting temperature, the resulting alloy typically does not effectively disperse over (or “wet”) the base material surfaces to be joined.
Nickel base boron-free braze alloys have also been proposed that contain Cr, Hf, Co, Zr (zirconium), Ti (titanium) and Al (aluminum) to produce a solidus temperature of about 1175 deg. C. That is, the temperature at which the solid material begins to liquefy, but only partially liquefy, (the solidus) is about 1175 deg. C. The mechanical properties, braze wettability, among other properties that might be achievable with such alloys are not known precisely, but it is expected that such properties are not suitable for wide gap structural brazing applications and effective extension of the braze filler by capillary wetting over broad surfaces.
Boron-free braze alloys have also been proposed with very high Hf or Zr content, for example by Miglietti, U.S. Pat. No. 6,520,401. Others have been proposed with very high Mn (manganese) content, for example by Laux et al “Fast Epitaxial High Temperature Brazing of Single Crystalline Nickel Base Superalloys”, Journal of Engineering for Gas Turbines and Power, Transactions of the ASME, Vol. 131, pp. 032102-1- 032102-8 (May 2009). However, the braze alloys proposed with very high Hf, Zr or Mn are substantially different in composition from the base alloy and, therefore, are expected to have properties quite different from those of the base alloy. Specifically nickel base alloys have been formulated without the intentional addition of Zr or with contents of Zr typically less than 0.1 weight percent. Other alloys have been formulated having Zr as high as about 1.5 weight percent for the purpose of possible grain refinement, that is to strengthen and improve ductility at grain boundaries with a view to increasing the creep-rupture strength of the material.
Thus, a need exists in the art for brazing procedures and materials that can be used for wide gap (which in some embodiments described herein is 1 mm or greater in some portion of the brazing zone) and structural repairs, without substantial degradation of the aforementioned component material properties (which in some exemplary embodiments include one or more of the properties identified in Table 2 herein), below 70% of the original base metal specifications.